Duplicates properties overwrite instead of merging, which is
inconsistent with how exhibit loads items.  Revised so that
second load will override if attributes are given explicitly in
the second occurrence, but will not reset attributes to default values
if they are missing in the second occurrence.
Added "id" as a core property in the exhibit database
Merge pull request #46 from karger/2013-09-29_properties

Fixed some handling of properties on data import and export. Exhibit was losing information when loading duplicate properties, and was not properly representing the "id" property as a built-in.